This invention relates to a drive or power system for a permanent-magnet (PM) synchronous motor. In recent years, such systems including dc to ac inverters have become increasingly important, and this interest has been due in part to improved magnetic materials for the permanent magnet rotors and to the reduced costs of solid state switching devices. Such a system has important characteristics that make them highly advantageous in some applications. For example, they are very efficient, simple in construction and dependable, capable of being powered by a dc source such as a solar array, and variable in speed so that the power output may be matched to the available power from the source.
In the past, synchronous motors have normally included a rotor starting cage or windings that are used to bring the motor up to or near synchronous speed. It is, however, desirable to be able to avoid the use of such a cage because it introduces harmonic losses during operation and, of course, a cage requires added expense. To operate such a motor without a starting cage, it is necessary to bring the motor up to rated speed by increasing the applied frequency from a low value to the normal value at a rate sufficiently low that the motor can remain in synchronism with the applied frequency.
However, it has been found that a PM synchronous motor without a starting cage, as described, is not stable except at the lower portion of a typical speed range. Ideally the rotor would turn at a uniform speed which is a function of the number of magnetic poles and the inverter applied frequency, but it has been found in practice that the rotor speed actually oscillates at its natural or harmonic frequency about the equilibrium speed. Such oscillations may be compared with the vibrations of a torsion spring at its harmonic frequency. When such a motor is started at a low speed and the speed is increased by increasing the applied frequency, the motor is stable (i.e., the oscillations are damped) until it reaches approximately one-fourth its rated speed, and then it becomes unstable. At this point the magnitude of the oscillations increases to the point where the rotor drops out of synchronism and then the motor stops. The speed at which this occurs depends on the motor construction and the type of load, and the motor and the load have inherent damping constants which prevent instability up to a certain point.
Three phase synchronous motors have, of course, been successfully operated in the past, and it is believed that such motors did not have such instability problems because they included either a starting cage or a starting motor, which functioned to damp such oscillations.
U.S. Pat. No. 3,753,063 of C. E. Graf describes a stabilizing system for damping speed oscillations of a three phase a-c reluctance-synchronous motor. According to this patent, the speed oscillations produce an a-c ripple current in the d-c bus supplying the inverter. The Graf patent describes a system for sensing the ripple current and modulating the inverter output voltage to counteract the oscillations. According to the patent, the modulation is such that an increase in rotor speed must result in an increase in the inverter output voltage, and vice versa. The precise construction of the a-c reluctance-synchronous motor is not clear from this patent, but the modulation system described in this patent would destabilize, not stabilize, a permanent magnet synchronous motor of the construction described herein.